Assisted low voltage write circuit

Abstract: A write circuit (28) in a magnetic disc drive (18) writes data to a magnetic disc (30) based on a write signal received (54). The write circuit includes a magnetic write head (80), an H-switch (78), and a headroom assistance circuit. The headroom assistance circuit includes a voltage divider (R1-R4), a capacitor (88), and a charging circuit (84, 86, 90) for charging and discharging the capacitor based on the write signals so that increased voltage appears across the magnetic recording write head during switching of the current polarity by the H-switch.

Description: The present invention relates to a write circuit for a disc drive. More particularly, the present invention relates to a headroom assistance circuit for an H-switch in a write circuit. A magnetic disc, for use in a computer disc drive, is a flat circular platter with a magnetic surface on which data can be stored by selective polarization of portions of the magnetic surface. The presence or absence of polarity transitions between the polarized portions represents particular binary values. The magnetic surface of the disc may be magnetized permanently with north/south or south/north orientation by applying a magnetic field near the disc. In a disc drive, the magnetic field for magnetizing the disc is created by a miniature horseshoe magnet which is suspended above the magnetically coated disc. The horseshoe magnet consists of a magnetic material surrounded by a conductive coil. The magnetic field is created by passing a current through the coil. The direction of the magnetic field is determined by the direction of the current through the coil. Thus, by changing the direction of the current through the coil, the host system can change the orientation of the magnetic field stored on the disc. The density of the data stored on the disc is affected by how quickly the host system can change the direction of the current through the coil. Since the coil is basically an inductor, the rate of change of the current is approximated by the following equation: V = Ldi dt or di dt = V L Where V is the voltage across the coil, L is the inductance of the coil, and di/dt is the rate of change of current through the coil with respect to time. Thus, it can be seen that the rate of change of current through the coil is dependent on the voltage across the coil. The current through the coil and the voltage across the coil are typically controlled by an H-switch connected to the magnetic write head. The H-switch consists of four transistors and various bias resistors. The transistors and resistors are balanced around the magnetic write head such that one transistor's emitter and another's collector are connected on each side of the magnetic write head. Depending upon the voltage applied to the H-switch, current flows from the emitter of one transistor through the magnetic write head and into the collector of another transistor on the opposite side of the magnetic write head. When the voltage applied to the H-switch is reversed, the current flows through a different pair of transistors in the opposite direction through the magnetic write head. The amount of voltage which can be placed across the magnetic write head is determined by the voltage supply available to the H-switch. Present desktop computers provide a 5 volt supply which is sufficient to maintain a quick rate of change for the write head current. However, portable computers require write circuits which use lower voltages. When lower voltage supplies are used with present H-switch technology, the time needed for current changes (rise time) is greater, which degrades overwrite performance at higher data rates. Thus, the performance of computers is restricted by current H-switch technology. A conventional H-bridge configuration is disclosed in e.g. IBM TDB, April 1981, Vol. 24, Nr. 11, on pages 5167-5168, PATENT ABSTRACTS OF JAPAN vol. 12, no. 173 (P-706)(3020) 24 May 1988, PATENT ABSTRACTS OF JAPAN vol. 13, no. 73 (P-830)2 0 February 1989 or PATENT ABSTRACTS OF JAPAN vol. 15, no. 466 (P-1280)26 November 1991. A wrile circuit according to the invention is contained in claim 1. Accordingly, the present invention is directed to a write circuit for a magnetic disc drive. The write circuit writes data to a magnetic disc based on a write signal received from the host system. The write circuit contains a magnetic recording write head, an H-switch, and a headroom assistance circuit. The headroom assistance circuit assists the H-switch in switching the current polarity through the magnetic recording write head. The headroom assistance circuit includes a voltage divider, a capacitor, and a charging circuit. The charging circuit rapidly changes the voltage on one side of the capacitor. Since the voltage across the capacitor changes slowly, this causes the voltage on the opposite side of the capacitor to be changed likewise. The voltage change on the opposite side of the capacitor is used by the H-switch to cause a greater voltage to appear across the recording write head during write current transitions. Embodiments of the present invention will now be described with reference to the accompanying drawings, of which: FIG. 1 is a block diagram of a write system of a disc drive; FIG. 2 is a more detailed block diagram of a write circuit used in the system of FIG. 1; FIG. 3 is a circuit diagram of a write circuit used in the system of FIG. 1; and FIG. 4 is also a circuit diagram of a write circuit used in the system of FIG. 1. 1. Overview FIG. 1 is a block diagram of a write system used in a disc drive. Write system 18 contains host 20, controller 22, encoder 24, write precomp circuit 26, write circuit 28, and magnetic disc 30. Host 20 is coupled to controller 22 and provides clock signal 42 and write data 44. Write data 44 is the binary data to be written to magnetic disc 30. Controller 22 is coupled to host 20 and encoder 24. Controller 22 receives write data 44 and clock 42 from host 20 and outputs controller data 46 and clock 45 to encoder 24. Controller 22 performs all functions necessary to receive data from host 20. Encoder 24 is coupled to controller 22 and write precomp circuit 26. Encoder 24 receives controller data 46 and clock 45 from controller 22 and outputs encoded data 50 and clock 49 to write precomp circuit 26. Write precomp circuit 26 is coupled to encoder 24 and write circuit 28. Write precomp circuit 26 receives encoded data 50 and clock signal 49 from encoder 24 and outputs precompensated data 54 to write circuit 28. Write precomp circuit 26 shifts pulses in encoded data 50 to compensate for peak shifting caused by pulse crowding or other pattern dependent transition shifting. Write circuit 28 is coupled to write precomp circuit 26 and is proximate magnetic disc 30. Write circuit 28 receives precompensated data 54 from write precomp circuit 26 and outputs magnetic field 58. Magnetic field 58 writes magnetic transitions on disc 30 which are indicative of the data to be stored on disc 30. Write circuit 28 changes the polarity of magnetic field 58 for each positive going edge in precompensated data 54. Magnetic disc 30 is proximate write circuit 28. A small portion of magnetic disc 30 resides within magnetic field 58 and is magnetized to the orientation of magnetic field 58. Magnetic disc 30 rotates relative to write circuit 28. This causes different portions of magnetic disc 30 to be in the proximity of write circuit 28 at different times. Changes in the orientation of magnetic field 58 causes different portions of magnetic disc 30 to have different magnetic orientations. These transitions in magnetic orientation on magnetic disc 30 represent data written to the disc. 2. Write Circuit 28 FIG. 2 is a more detailed block diagram of write circuit 28 of FIG. 1. Write circuit 28 comprises delay 72, flip-flop 74, H-switch controller 76, H-switch 78, write head 80, one-shot 84, charging switch 86, large capacitor 88, and diode 90. Delay 72 is coupled to write precomp circuit 26 of FIG. 1, one-shot 84 and flip-flop 74. Delay 72 receives precompensated data 54 and outputs delayed data signal 100 to flip-flop 74. Delay 72 delays the positive edges of precompensated data 54 to properly time the switching of large capacitor 88 with the switching of the H-switch 78. This will be explained in greater detail later in the specification. Flip-flop 74 is coupled to delay 72 and H-switch controller 76. Flip-flop 74 receives delayed data 100 from delay 72 and outputs flip-flop output 102 to H-switch controller 76. Flip-flop 74 creates flip-flop output 102 by forming a digital signal with a transition for each positive going transition in delayed data signal 100. H-switch controller 76 is coupled to flip-flop 74, H-switch 78, and common reference 83. H-switch controller 76 receives flip-flop output 102 from flip-flop 74 and provides differential input voltage signal 104 to H-switch 78. H-switch controller 76 produces differential input voltage signal 104 by dividing the voltage of flip-flop output 102. Also, the common mode voltage of differential input voltage signal 104 is affected by common reference 83. This allows common reference 83 to control the biasing of H-switch 78. H-switch 78 is connected to H-switch controller 76, write head 80, and common reference 83. H-switch 78 receives differential input voltage signal 104 from H-switch controller 76 and provides write head voltage 108 and write head current 106 to write head 80. H-switch 78 produces write head voltage 108 and write head current 106 based on differential input voltage signal 104. The polarity of the differential input voltage signal 104 determines the polarity of write head voltage 108 and the direction of write head current 106. Write head 80 is connected to H-switch 78 and is proximate magnetic disc 30 of FIG. 1. Write head 80 receives write head voltage 108 and write head current 106 from H-switch 78 and produces magnetic field 58. Magnetic field 58 is proportional to write head current 106. Magnetic field 58 is used to magnetize portions of magnetic disc 30 of FIG. 1. One-shot 84 is connected to write precomp circuit 26 of FIG. 1, delay 72, and charging switch 86. One-shot 84 receives precompensated data 54 from write precomp circuit 26 and provides one-shot signal 110 to charging switch 86. One-shot signal 110 contains one pulse for each positive going transition in precompensated data 54. Charging switch 86 is connected to one-shot 84 and large capacitor 88. Charging switch 86 receives one-shot signal 110 from one-shot 84 and provides capacitor voltage signal 112 for large capacitor 88. For each pulse received from one-shot 84, charging switch 86 quickly changes capacitor voltage 112 from the supply voltage to ground. Large capacitor 88 is connected to charging switch 86 and common reference 83. In steady state, common reference 83 is at a positive reference potential equal to the diode on-voltage across diode 90. When capacitor voltage 112 changes from the supply voltage to ground, common reference 83 is driven substantially below ground, until current flowing from H-switch 78 to common reference 83 has accumulated significant charge in large capacitor 88. Ideally, the voltage at common reference 83 is pulled down, below its nominal potential, by an amount equal to the power supply voltage. However, the resistance of the charging switch 86 and the large capacitor 88 will typically reduce the actual voltage change measured at common reference 83. Diode 90 is connected to common reference 83 and ground. Diode 90 blocks the flow of current from ground into common reference 83. This assures that when capacitor voltage 112 changes from the supply voltage to ground, the current drawn to large capacitor 88 does not come from ground, but rather from the H-switch controller 76 and the H-switch 78. As shown by this description, the present invention uses one-shot 84, delay 72, charging switch 86, large capacitor 88, diode 90, and H-switch controller 76 to develop a larger voltage drop across the magnetic write head at transition. This voltage drop is created when the buffered one-shot, one-shot 84 together with charging switch 86, drives the capacitor voltage on capacitor 88 from the supply voltage to ground and thereby drives the common reference 83 below ground. The use of the large capacitor 88 to give a momentary increase in voltage across the magnetic write head 80 allows lower voltage power supplies to be used in disc drives while maintaining the switching time of the write current in the magnetic write head. 3. Write Circuit 28 In Detail FIG. 3 is a more detailed circuit diagram of write circuit 28 in the system of FIGs. 1 and 2. Elements shown in FIG. 3 that are similar to those shown in FIG. 2 are correspondingly numbered. a. Operation Of Delay 72 And Flip-flop 74 Flip-flop 74 is a D flip-flop and has data input D, output Q, inverse output, and clock input CK. Data input D is connected to inverse output Q and clock input CK is connected to the output of delay 72. Clock input CK receives delayed data signal 100 from delay 72. Inverse output Q is connected to data input D and H-switch controller 76. Output Q is connected to H-switch controller 76. Delay 72 delays the precompensated data 54 to flip-flop 74. D flip-flop 74 uses the delayed data signal 100 as its clock signal. A positive going transition in the clock signal will cause the output Q to switch to the value at data input D. The two possible values for output Q and inverse output Q are a logical high voltage, V Q , and a logical low voltage, nearly zero. In addition, output Q and inverse output Q are never the same value. Since data input D is connected to the inverse output Q , the value at data input D just before transition will always be the opposite of output Q. Thus, each time the clock input CK receives a positive going pulse, output Q switches to it complimentary value. Therefore, every transition in output Q represents a positive going pulse in precompensated data 54. b. H-switch controller 76, H-switch 78, and Write head 80 In Detail H-switch controller 76 includes resistors R1 and R2, diodes D1 and D2, and resistors R3 and R4. H-switch 78 includes resistors R5, R6, R7, R8 and transistors Q1, Q2, Q3, Q4, Q5, and Q6. Resistor R1 of H-switch controller 76 has a first side and a second side. The first side is connected to inverse output Q of flip flop 74. The second side is connected to the anode of diode D2 and the bases of transistors Q2 and Q4 of H-switch 78. Resistor R2 has a first side and a second side. The first side is connected to output Q of flip flop 74. The second side is connected to the anode of diode D1, and the bases of transistors Q1 and Q3 of H-switch 78. Diode D1 has an anode and a cathode. The anode is connected to resistor R2 and the bases of transistors Q1 and Q3. The cathode is connected to resistor R4. Diode D2 has an anode and a cathode. The anode is connected to resistor R1 and the bases of transistors Q2 and Q4. The cathode of diode D2 is connected to resistor R3. Resistor R4 has a first side and a second. The first side is connected to the cathode of diode D1. The second side is connected to common reference 83. Resistor R3 has a first and a second side. The first side is connected to the cathode of diode D2. The second side is connected to common reference 83. Resistors R1 and R2 have identical resistances. Resistors R3 and R4 also have identical resistances although probably different from resistors R1 and R2. Resistor R7 has a first side and a second side. The first side is connected to power supply V S . The second side is connected to the base of transistor Q6 and the collector of transistor Q1. Transistor Q6 has a base, a collector and an emitter. The base of transistor Q6 is connected to resistor R7 and the collector of transistor Q1. The collector is connected to power supply V S and the emitter is connected to write head 80 and the collector of transistor Q3. Resistor R8 has a first side and a second side. The first side is connected to power supply V S . The second side is connected to the base of transistor Q5 and the collector of transistor Q2. Transistor Q5 has a base, a collector and an emitter. The base is connected to resistor R8 and the collector of transistor Q2. The collector is connected to power supply V S and the emitter is connected to write head 80 and the collector of transistor Q4. Transistor Q1 has a base, a collector and an emitter. The base is connected to the base of Q3, the anode of diode D1 and resistor R2. The collector is connected to resistor R7 and the base of transistor Q6 and the emitter is connected to resistor R5 and the emitter of transistor Q2. Transistor Q3 has a base, a collector and an emitter. The base is connected to the base of transistor Q1, the anode of diode D1 and resistor R2. The collector is connected to the emitter of transistor Q6 and write head 80, and the emitter is connected to the emitter of transistor Q4 and resistor R6. Transistor Q2 has a base, a collector and an emitter. The base is connected to the base of transistor Q4, the anode of diode D2, and the resistor R1. The collector is connected to resistor R8 and the base of transistor Q5 and the emitter is connected to resistor R5 and the emitter of transistor Q1. Transistor Q4 has a base, a collector and an emitter. The base is connected to the base of transistor Q2, the anode of diode D2, and resistor R1. The collector is connected to write head 80 and the emitter of transistor Q5. The emitter is connected to the emitter of transistor Q3 and resistor R6. Resistor R5 has a first side and a second side. The first side is connected to the emitters of transistors Q1 and Q2. The second side is connected to common reference 83. Write head 80 has a first side and a second side. The first side is connected to the emitter of transistor Q6 and the collector of transistor Q3. The second side is connected to the emitter of transistor Q5 and the collector of transistor Q4. The resistance of write head 80 is represented by resistor R9 and its inductance is represented by inductor L1 Resistor R6 has a first side and a second side. The first side is connected to the emitters of transistors Q3 and Q4. The second side is connected to common reference 83. c. Operation Of H-switch controller 76, H-switch 78, and Write head 80 The resistor pairs R1, R3 and R2, R4 act as a voltage divider to divide the voltage provided between the output 102 of flip flop 74 and common reference 83. Diodes D1 and D2 each provide one diode on-voltage when current is flowing through them, which is intended to match the individual base-emitter on-voltages of transistors Q1, Q2, Q3, and Q4. The divided voltage is provided as the differential voltage input signal 104 to H-switch 78. Resistor pairs R1, R3 and R2, R4 are chosen such that the voltage across resistor R6 in H-switch 78, in steady state, is above a minimum voltage VMIN. This value is typically around .35 volts. This minimum voltage reduces current variations through the write head 80 due to temperature changes. Depending on whether Q is in a logic high state or inverse output Q is in a logic high state, significant current will flow through either resistors R2 and R4, diode D1, and transistor Q3 or through resistors R1 and R3, diode D2 and transistor Q4, but not through both sets of circuit elements simultaneously. Depending on which set of elements is conducting current, the voltage drop across the base emitter junction of Q3 or Q4, is approximately equal to the diode on-voltage across diode D1 or diode D2. Thus, the voltage across resistor R3 or R4 is approximately equal to the voltage across resistor R6. Therefore, to ensure that the voltage across resistor R6 is at or above V MIN , resistor pairs R1, R3 and R2, R4 should be chosen such that the steady state on-voltage across resistors R3 and R4 is at or above V MIN . Since the ratios of resistor R1 to resistor R3 and resistor R2 to R4 are equal to the ratio of the on-voltage across resistor R1 to the on-voltage across resistor R3, the values for resistors R1, R3 and R2, R4 are chosen from the following equation: R1 R3 = R2 R4 = V Q -V D -V 90 -V MIN V MIN Where: V Q equals the high voltage from flip flop 74; V D equals the on-voltage across diode D1 (or D2); V 90 equals the voltage drop across diode 90; and V MIN is the minimum voltage needed across resistor R6. In the preferred embodiment, the resistance of resistor R1 is equal to the resistance of resistor R2, and the resistance of resistor R3 is equal to the resistance of resistor R4. When output Q of flip flop 74 is at V Q , inverse output Q is close to zero. The voltage at the anode of diode D1 is substantially greater than zero, and the voltage at the anode of diode D2 is close to zero. The voltage at the anode of diode D1 causes transistors Q1 and Q3 to turn on. A lack of positive voltage at the anode of diode D2 causes transistors Q2 and Q4 to remain off. When transistor Q1 turns on, it draws current through resistor R7. The current flows through resistor R7 and causes a voltage drop across resistor R7. This causes the voltage at the collector of transistor Q1 to drop below the voltage needed to turn on transistor Q6. Therefore, transistor Q6 is off. Since transistors Q2 and Q4 are both off, no current flows into the collector and out the emitter of either of these transistors. With no current flow through transistor Q2 there is very little current flow through resistor R8. This causes the voltage at the collector of transistor Q2 to be nearly at the voltage supply V S . Since this voltage is at the base of transistor Q5, transistor Q5 is on. The combination of having both transistors Q3 and Q5 on and transistors Q4 and Q6 off causes current to flow into the collector and out of the emitter of transistor Q5, through write head 80, into the collector of transistor Q3 and out of the emitter of transistor Q3. This current finally passes through resistor R6 and diode 90. When output Q of flip flop 74 is nearly zero, inverse output Q is at V Q , the voltage at the anode of diode D2 is substantially greater than zero, and the voltage at the anode of diode D1 is close to zero. The voltage at the anode of diode D2 causes transistors Q2 and Q4 to turn on. A lack of positive voltage at the anode of diode D1 causes transistors Q1 and Q3 to remain off. When transistor Q2 turns on, it draws current through resistor R8. The current flows through resistor R8 and causes a voltage drop across resistor R8. This causes the voltage at the collector of transistor Q2 to drop below the voltage needed to turn on transistor Q5. Therefore, transistor Q5 is off. Since transistors Q1 and Q3 are both off, no current flows in the collector and out the emitter of either of these transistors. With no current flow through transistor Q1 there is very little current flow through resistor R7. This causes the voltage at the collector of transistor Q1 to be nearly at the supply voltage V S . Since this voltage is at the base of transistor Q6, transistor Q6 is on. The combination of having both transistors Q4 and Q6 on and transistors Q3 and Q5 off causes current to flow into the collector and out of the emitter of transistor Q6, through write head 80, into the collector of transistor Q4 and out of the emitter of transistor Q4. This current finally passes through resistor R6 and diode 90. d. One-shot 84 In Detail One-shot 84 includes D flip-flop F2, capacitor C2, and resistor R10. D flip-flop F2 has data input D, clock input CK, reset input R, and output Q. Clock input CK is connected to write precomp circuit 26 and receives precompensated data 54. Input D is connected to a logic high voltage H. Output Q is connected to resistor R10 and charging switch 86. Reset input R is connected to capacitor C2 and resistor R10. Capacitor C2 has a first side and a second side. The first side is connected to resistor R10 and reset input R of D flip-flop F2. The second side is connected to ground. Resistor R10 has a first side and second side. The first side is connected to reset input R of D flip-flop F2 and capacitor C2. The second side is connected to output Q of D flip-flop F2 and charging switch 86. e. Operation Of One-shot 84 A pulse in precompensated data 54 is received by clock input CK of D flip-flop F2. The pulse enables flip-flop F2 and output Q switches to the logic high voltage value of input D. Reset input R attempts to change to a digital value of one (a logic high), but is hindered by resistor R10 and capacitor C2. When reset input R does reach a digital value of one, flip-flop F2 is reset, and output Q switches to a digital value of zero. Thus, one positive going pulse is created on output Q for each positive edge in precompensation data 54. The duration of this pulse is proportional to the (R10 * C2) time constant. f. Charging Switch 86, Large Capacitor 88, And Diode 90 In Detail Charging switch 86 has an input connected to the output Q of D flip-flop F2 and resistor R10, and an output connected to large capacitor 88. Charging switch 86 also has connections to the power supply V S and ground. Large capacitor 88 has a first side and a second side. The first side is connected to the charging switch 86 and has a voltage referred to as the capacitor voltage 112. The second side is connected to common reference 83 and diode 90. Diode 90 (which is preferably a Schottky diode) has an anode and a cathode. The cathode is connected to ground and the anode is connected to common reference 83 and large capacitor 88. g. Operation Of Charging Switch 86, Large Capacitor 88 And Diode 90 The one-shot signal from one-shot 84 has two states: pulse and non-pulse. In the non-pulse state, the charging switch 86 keeps the capacitor voltage 112 near the power supply voltage V S . When one-shot signal 110 switches to a pulse state, charging switch 86 quickly reduces the capacitor voltage 112 to ground. This causes large capacitor 88 to drive the common reference 83 below ground. Ideally, the voltage at common reference 83 is pulled down, below its nominal potential, by an amount equal to the entire capacitor voltage change (i.e., the power supply voltage). However, the resistance of charging switch 86 and large capacitor 88 will typically reduce the actual voltage change measured at common reference 83. When one-shot signal 110 switches back to a non-pulse state, the charging switch 86 quickly resets the capacitor voltage 112 to close to the supply voltage V S . This causes large capacitor 88 to reset common reference 83 to its nominal value of V 90 . Since the input signal to the H-switch 78 is delayed by delay 72, the described voltage change at common reference 83, begins just before the transition of current in the H-switch. This allows H-switch 78 to provide a larger voltage across the write head during write current transitions, which reduces the switching time of the write current. This allows computers to use lower voltage power supplies, which decreases total power consumption, and permits the realization of smaller computers. FIG. 4 shows the preferred embodiment of the write circuit 28 shown in FIG. 2. This write circuit operates in the same manner as the circuit in FIG. 3, except that the H-switch controller 76 uses two voltage dividers to control the H-switch 78, with one of the voltage dividers being clamped. The first voltage divider in H-switch controller 76 is the same as the divider found in FIG. 3. The only difference in FIG. 4 is that resistor R1 is now only connected to diode D2 and the base of transistor Q2. In addition, resistor R2 is only coupled to diode D1 and the base of transistor Q1. This means that the resistor pairs R1, R3 and R2, R4 only control the H-switch biasing transistors Q1 and Q2. The new clamped voltage divider consists of resistors R11, R12, R13, R14, R15, and R16, and diodes D3, D4, D5, and D6. Resistor R15 is connected to the Q output of flip-flop 74, diode D3 and resistor R11. Resistor R16 is connected to diode D5, resistor R13 and the inverse output Q of flip-flop 74. Diode D3 is connected to resistors R15 and R11 and diode D4. Diode D4 is connected to common reference 83 and diode D3. Diode D5 is connected to resistors R16 and R13 and diode D6. Diode D6 is connected to common reference 83 and diode D5. Resistor R11 is connected to resistors R15 and R12, diode D3 and the base of transistor Q3. Resistor R12 is connected to common reference 83, resistor R11 and the base of transistor Q3. Resistor R13 is connected to resistors R16 and R14, diode D5 and the base of transistor Q4. Resistor R14 is connected to common reference 83, resistor R13 and the base of transistor Q4. h. Operation Of The Clamped Voltage Divider The clamped voltage divider is a balanced device so that the interaction and operation of resistors R15, R11, R12, and diodes D3 and D4 are the same as the interaction and operation of resistors R16, R13, R14, and diodes D5 and D6. Therefore, only the operation of resistors R15, R12, R11, and diodes D3 and D4 will be described. Resistor R12 is chosen such that its resistance is equal to the resistance of resistor R15. Resistor R11 is chosen such that its resistance is one third the resistance of resistor R15. This relationship causes the voltage across R6 to remain at a nearly constant value of one half of the individual diode on-voltage associated with diodes D3, D4, D5, and D6. This can be seen from the fact that, because resistor R11 is one third of the value of resistor R12, and because there are a total of two diode on-voltages across resistors R11 and R12, resistor R11 will have a voltage of 1/2 of a diode on-voltage across it and resistor R12 will have a voltage of 1 1/2 diode on-voltages across it. Assuming that there is a voltage of one diode on-voltage across the base-emitter junction of transistor Q3, that forces the voltage across resistor R6 to be one half of a diode on-voltage. This arrangement operates so that the voltage across resistor R6 is not dependant on the voltage of common reference 83, as it was for the circuit in FIG. 3. In the embodiment of FIG. 3, a portion of the voltage change that appears at common reference 83 also appears across resistor R3 or R4, which directly increases the voltage across both resistors R5 and R6. The increased voltage across resistor R5 increases the voltage swing at the bases of transistors Q5 and Q6, which would otherwise directly limit the switching voltage applied across write head 80. However, the increased voltage across resistor R6 reduces the H-switch headroom directly, and increases the magnitude of the write current, slightly before, during, and slightly after each write current transition. This write current increase does not provide performance benefits, but reduces the effectiveness of large capacitor 88 by charging it faster than necessary. Thus, not all of the voltage change from switching capacitor 88 is actually applied across write head 80. However, the clamped voltage divider shown in FIG. 4 does not allow the voltage across resistor R6 to increase. Rather, it remains nearly constant at approximately one half of a diode on-voltage. Thus, the entire voltage change caused by switching capacitor 88, at common reference 83, is applied across write head 80. This further increases the performance of the capacitor assisted low voltage write circuit. CONCLUSION The present invention develops an increased voltage across the magnetic write head during a transition in the write current signal applied to the write head. This increased voltage is created by switching a capacitor voltage so that the common reference voltage of the circuit is momentarily driven below ground. This momentary increase in voltage across the magnetic write head allows the disc drive system to use a lower voltage power supply, while maintaining the switching time of the write current in the magnetic write head. Using lower voltage supplies saves space, power, and money. Although the present invention has been described with reference to preferred embodiments, persons skilled in the art will recognise that changes may be made in form and detail without departing from the scope of the present invention.

Claims: A write circuit (28) in a magnetic disc drive (18) for receiving a write signal (54) and for writing data based on the write signal to a magnetic disc (30), the write circuit comprising: a magnetic recording write head (80); an H-switch (78), connected to the magnetic recording write head for switching current polarity through the magnetic recording write head based on the write signal; and characterised by a headroom assistance circuit, connected to the H-switch, for assisting the H-switch in switching the current polarity through the magnetic recording write head, the headroom assistance circuit, comprising: a voltage divider (R1-R4) connected to the H-switch for controlling the voltage across the magnetic recording write head based on the write signal; a capacitor (88) for connecting a common reference node (83) of the voltage divider and the H-switch with charging (84, 86, 90) means for charging and discharging the capacitor, based on the write signal to increase the voltage appearing across the magnetic recording write head during switching of the current polarity by the H-switch. A write circuit as claimed in claim 1, wherein the H-switch includes: write head current driver means connected to the magnetic recording write head, for driving current through the magnetic recording write head; and write head current control means connected to the write head current driver means, for controlling current through the magnetic recording write head. A write circuit as claimed in claim 1 or 2, wherein the voltage divider includes: write signal translator means (72,74) for translating the write signal and producing a translated write signal; and divisional means (R 1 -R 4 ) connected to the write signal translator means and the H-switch, for dividing the translated write signal to control the voltage across the magnetic recording write head, and to reduce current variations in the magnetic recording write head when the H-switch is not being switched. A write circuit as claimed in claim 3, wherein the divisional means includes: voltage clamping means (D 1 ,D 2 ) for clamping a voltage across a portion of the H-switch at a fixed level so a greater portion of the increased voltage is applied across the magnetic recording write head during switching. A write circuit as claimed in claim 3, wherein the write signal translator means includes: delay means (72) for delaying the write signal and producing a delayed write signal; and a flip-flop (74) connected to the delay means, producing a translated write signal in response to the delayed write signal. A write circuit as claimed in any of the preceding claims, wherein the charging means includes: one-shot means (84) for providing a one-shot signal based on the write signal; a switch (86) connected to the one-shot means and the capacitor, for switching voltage on one side of the capacitor based on the one-shot signal; and ground current restriction means (90), connected to the capacitor, for restricting current flow from ground. A write circuit as claimed in claim 6, wherein the one-shot means includes: a write assistance one-shot for providing one pulse for each positive edge in the write signal. A write circuit as claimed in claim 6, wherein the ground current restriction means includes a Schottky diode. A write circuit (28) in a disc drive (18) as claimed in any of the preceding claims when dependent upon claim 5, in which the write circuit further includes H-switch control means (76), connected to the flip-flop, producing at least one H-switch control signal based on the translated write signal. A write circuit (28) as claimed in any of the preceding claims, in which said H-switch (78) has a first terminal and a second terminal and being responsive to the changes in voltage values to change between a first mode and a second mode, the H-switch providing a write head current (106) having alternatively a first flow direction from the first terminal to the second terminal when the H-switch is operating in the first mode and a second flow direction from the second terminal to the first terminal when the H-switch is operating in the second mode; the magnetic write head (80) is electrically coupled between the first and second terminals of the H-switch and responsive to the write head current to provide the differing magnetic fields, the write head current establishing a voltage drop (108) at the write head relative to a voltage at a reference node (83); the capacitor (88) has a first terminal and a second terminal, the second terminal being electrically coupled to the reference node; and the charging control means (84, 86) electrically coupled to the first terminal of the capacitor for controlling charging and discharging of the capacitor based on the differing voltage values of the write signal to provide voltage pulses to selectively change the voltage of the reference node. A write circuit as claimed in claim 10 wherein discharging the capacitor changes the voltage at the reference node to increase the voltage drop at the write head relative to the voltage at the reference node during the transitions between the first and second flow directions of the write head current. A write circuit as claimed in claim 11 wherein the H-switch has a resistive portion having two terminals such that one of the terminals of the resistance portion is electrically coupled to the reference node and wherein an increase in voltage drop across the two terminals of the resistive portion reduces the increase of the voltage drop at the write head relative to the voltage at the reference node during transitions between the first and second flow directions of the write head current and wherein the voltage clamping means is electrically coupled to the H-switch for stabilising the voltage drop across the two terminals of the resistive portion to allow a greater increase in the voltage drop at the write head relative to the voltage at the reference node during transitions between the first and second flow directions of the write head current. A write circuit as claimed in any one of the preceding claims when dependent upon claim 6 wherein a power supply supplies the write circuit with a power supply voltage and wherein: the one-shot means provides a one-shot signal having a stable voltage state and a pulse voltage state, the one-shot signal changing from the stable state to the pulse state based on the differing voltage values of the write signal, the one-shot signal staying on the pulse state for a selected duration; and the voltage switch (86) is responsive to the one-shot signal to apply a capacitor voltage to the first terminal of the capacitor, the value of the capacitor voltage being approximately at the level of the power supply voltage when the one-shot signal is in the stable state, the voltage switch changing the value of the capacitor voltage to a ground voltage level when the one-shot signal changes to the pulse state.

Inventors: MADSEN TIMOTHY A, MADSEN, TIMOTHY A.
Applicants: SEAGATE TECHNOLOGY, SEAGATE TECHNOLOGY INTERNATIONAL
Application year: 1994
Main classification: G11B5
IPC codes: G11B5